The present invention relates to devices that removes dust particles from gases, and more specifically to a particular category of such devices called wet scrubbers, and more specifically to wet scrubbers with nozzles, and even more specifically to wet scrubbers with self-cleaning nozzles.
The concern of this invention is with air pollution resulting from the emission into the atmosphere of particulate matter and other contaminants. As noted in the Federal Register of Dec. 27, 1996 (Vol. 61, No. 25), xe2x80x9c[t]he primary goal of the Clean Air Act is to enhance the quality of the Nation""s air resources and to promote the public health and welfare and the productive capacity of its population.xe2x80x9d
Particulate matter is made up of tiny particles in the atmosphere that can be solid or liquid (except for water or ice) and is produced by a wide variety of natural and manmade sources. Particulate matter includes dust, dirt, soot, smoke and tiny particles of pollutants. Some particles attract and combine with amounts of water so small that they do not fall to the ground as rain. Major sources of particulate pollution are factories, power plants, trash incinerators, motor vehicles, construction activity, fires, and natural windblown dust. Particles below 10 microns in size (about seven times smaller than the width of a human hair) are more likely to travel deep in the respiratory system, and be deposited deep in the lungs where they can be trapped on membranes. If trapped, they can cause excessive growth of fibrous lung tissue, which leads to permanent injury. Children, the elderly, and people suffering from heart or lung disease are especially at risk. Particles of 10 microns or less are also referred to as PM10.
Electrostatic precipitators, which have been used for particulate control since 1923, use electrical fields to remove particulate from boiler flue gas. Inasmuch as precipitators act only on the particulate to be removed, and only minimally hinder flue gas flow, they have very low-pressure drops, and thus low energy requirements and operating costs. The main drawback of a conventional electrostatic precipitator, apart from requiring a high voltage supply, is that the contaminants collected on the collecting electrode tend to adhere thereto. This makes it necessary, on occasion, to shut down the precipitator in order to scale off the accumulated dirt.
Fabric filter collectors are conceptually simple: by passing flue gas through a tightly woven fabric, particulate in the flue gas will be collected on the fabric by sieving and other mechanisms. The dust cake, which forms on the filter from the collected particulate, can contribute significantly to collection efficiency. Practical application of fabric filters requires the use of a large fabric area in order to avoid an unacceptable pressure drop across the fabric.
Cyclones use centrifugal force to separate particulate from gas streams, and belong to the broader family of mechanical collectors, which use a variety of mechanical forces to collect particulate. A multiple cyclone is an array of a large number of small (several inch diameter) cyclones in parallel. Multiple cyclones have overall mass removal efficiencies of 70-90%. However, cyclone collection efficiencies fall off rapidly with particle size, so that control of fine particulate (PM-2.5) is limited.
Wet scrubbers are based on the collection of particles in liquid droplets, and scrubber design therefore is optimized for droplet creation. In venturi scrubbers, which are commonly used for particulate collection, the scrubbing liquid and flue gases accelerate through a converging section of duct into a narrow throat, and then pass through the throat into a diverging section. In the throat, very high gas velocity shears the scrubbing liquid into a cloud of very fine droplets, which collect particles.
The water droplets are thoroughly mixed with the dirty air, and the solid pollutants in gas air are thoroughly wetted. The wet solid pollutants, being relatively heavy, are deflected downward through the action of the baffles and reduced air velocities into the water reservoir. A constant-volume air control means interposed between the chamber and the source of polluted air, such as dirty air from an incinerator or the like, contains a primary air inlet duct and a secondary air inlet duct where the air through each duct is controlled by a damper. The dampers of the two ducts are coupled to operate complementary to each other to provide a constant air flow to the chamber for various flows of polluted air directed to the scrubber through the primary air inlet duct.
Wet Scrubber systems dealing with acid Gas/SO rely on a chemical reaction with a sorbent to remove a wide range of pollutants, including sulfur dioxide (SO2), acid gases, and air toxins, from flue gases. When used to remove or xe2x80x9cscrubxe2x80x9d SO2 from the flue gas, these devices are commonly called flue gas desulfurization (FGD) or scrubber systems. FGD or scrubber systems are generally classified as either xe2x80x9cwetxe2x80x9d or xe2x80x9cdryxe2x80x9d. Wet scrubbers are increasingly recognized as an important part of a multi-pollutant control program. In a wet scrubber, a liquid sorbent is sprayed into the flue gas in an absorber vessel. The gas phase or particulate pollutant comes into direct contact with a sorbent liquid and is dissolved or diffused (scrubbed) into the liquid. The liquid interface for gas and particle absorption include liquid sheets, wetted walls, bubbles and droplets.
In the wet processes, a wet slurry waste or by-product is produced. Most wet FGD systems use alkaline slurries of limestone or slaked lime as sorbents. Sulfur oxides react with the sorbent to form calcium sulfite and calcium sulfate. Uptake of the pollutant by the sorbent results in the formation of a wet solid by-product that may require additional treatment, or when oxidized, results in a gypsum by-product that can be sold. Scrubber technologies for wet scrubbing of gaseous pollutants can achieve extremely high levels of multi-pollutant control, including acid gases, SO2, fine particulates and heavy metals (e.g., mercury) from utility and industrial coal-fired boilers, waste-to-energy systems, and other industrial processes.
Wet scrubber technology can be applied to difficult processes such as gas absorption and particle collection, treating combustible particles, and removal of wet, sticky or corrosive particles. Wet scrubbers are used in industrial process mercury removal and to remove ionic forms of mercury from the gas stream of coal-fired power generation facilities. Wet scrubbers generally have relatively small space requirements, low capital cost, and are able to process high temperature, high acidity, and high humidity flue gas streams. Scrubber costs have continued to decrease, largely because of technical innovations. Scrubber energy requirements have also continued to decrease, helping to lower operating costs.
FGD systems are an increasingly significant part of a multi-pollutant control approach, even as the energy requirements of these systems are decreasing to where these systems now consume only about 1% of total boiler output. Where low-cost high-sulfur fuels are available, or where the required reductions are very high, scrubbing is often a viable control option. New wet scrubbers routinely achieve SO2 removal efficiencies of 95%, with some scrubbers achieving removal efficiencies of up to 99%. Scrubbers have been used in the EPA Acid Rain Program on coal-fired boilers, which are significant sources of hydrochloric acid (HCl) and hydrofluoric acid (HF).
According to the EPA and others sources, both wet and dry scrubbers have been shown to reduce HCl emissions by 95% and more, and wet scrubbers have been shown to reduce HF emissions by more than one-third. Others have reported ranges of 87-94% removal of chlorine and 43-97% removal of fluorine by both wet and dry scrubbers. In addition, wet scrubbers also provide significant removal of arsenic, beryllium, cadmium, chromium, lead, manganese, and mercury from flue gas. Wet scrubbers can be generally grouped by geometric designs and method for gas-liquid contact. Several groupings include packed-bed, counter-flow, cross-flow, bubble-plate, open spray (single and double loop) tower, dual-flow tray, cyclonic, and venturi designs. However, there are many proprietary systems designed around specific industry needs. Design and operating parameters include scrubber geometrical shape, liquid spray or injection locations, gas residence time, gas velocities, gas and liquid temperatures, gas and liquid pressure drop, and, liquid/gas flow rate ratio.
Of prior art interest are the wet disclosed in the Sibley et al. U.S. Pat. No. 4,609,386. In the Sibley et al scrubber, pollutants are removed from a gaseous or air stream by passing the polluted air through a scrubber chamber in a tortuous path, the flowing air being contacted by water sprayed into the chamber by nozzles. This action causes pollutants to be transferred from the polluted air to the water, thereby cleaning the air. Although the wet scrubbers in the patents above is highly effective in removing contaminants from the gaseous stream being discharged into the atmosphere, it does not overcome the problem of the wet scrubber being clogged by pollutants, fragments and other byproducts of the waste gas cleaning process. As a result, the nozzles becomes clogged which leads to a decrease in efficiency of the wet scrubber and increase in the air pollution. Furthermore, the EPA policy requires a shutdown of the system in order to remove the pollutants from the wet scrubber. This leads to frequent interruptions and an increase in production, labor, and maintenance costs.
Therefore, the object of the present invention is to provide a method of waste gas treatment capable of cleaning nozzles of a wet scrubber that does not require shutting down the whole system.
Another object of the present invention is to provide a wet scrubber with nozzles capable of self-cleaning when clogged with pollutant particles.
One more object of the present invention is to provide a nozzle with the quality of self-cleaning when clogged.
According to the present invention, a method of waste gas treatment with the use of a wet scrubber conventionally provides for pumping water under pressure through a pipe into at least one nozzle, passing waste gases through out of the scrubber, and spraying water in the scrubber being the nozzle to wet envelope pollutant particles in the waste gases preventing them from being discharged into atmosphere. The method additionally provides for monitoring the pressure in the pipe, and, upon exceeding by the pressure a preset value due to clogging an outlet orifice of the nozzle by the pollutants, causing the outlet orifice to increase. Due to that, the pollutant particles fall down the outlet orifice unclogging the orifice.
A wet scrubber according to the present invention is composed of a scrubber body, in which at least one nozzle is installed, a water reservoir and a pipe, through which a pump pumps water from the reservoir into the nozzle. The scrubber also comprises means monitoring water pressure in the pipe, control means controlled by the monitoring means when the water pressure exceeds a preset value due to clogging of the nozzle by pollutant particles, and means unclogging the nozzle in response to a signal from the control means.
The monitoring means may include pressure sensor/transmitter, the control means may include a control panel generating an electric signal to the nozzle upon receiving a signal from the sensor/transmitter of the water pressure in the pipe exceeding a preset value, and the unclogging means includes means enlarging an outlet orifice of the nozzle.
Those means enlarging the outlet orifice include an electromagnetic coil in the nozzle and making the nozzle with the structure where the outlet orifice is formed as an outwardly diverging conical bore in a bottom of a body of the nozzle, in which bore a coaxial conical member is installed. In this way, upon energizing the electromagnetic coil the body moves toward the coil and the distance between walls of the conical bore and the coaxial conical member defining the outlet orifice increases enlarging the outlet orifice, whereby the pollutant particles fall down the outlet orifice unclogging it. Otherwise, the body may be stationary, and the coil or a portion thereof moves toward the body with the same effect.
A self-cleaning nozzle according to the present invention is proposed to be made comprising a body defined by a top, a bottom, and walls, an inlet in the wall, through which inlet water is supplied into the nozzle, an outwardly diverging conical bore in the bottom, in which bore a coaxial conical member is installed. An outlet orifice, through which water is sprayed out of the nozzle, is defined by a distance between walls of the bore and the coaxial conical member. Also provided is an electromagnetic coil, the coaxial conical member being rigidly affixed to the coil, and the body having a spring-biased connection to the coil. Thanks to this structure, when the coil is energized, the body is attracted towards the coil compressing the spring, the distance between walls of the bore and the coaxial conical member increases, and pollutant particles clogging the outlet orifice are caused to fall down the outlet orifice unclogging the orifice. Alternatively, the body may be stationary and fixed to the scrubber, for example via lugs, and the coil structure or a portion thereof such as armature moves toward the body with the same effect.
The nozzle may also comprise a coil connector, to which the electromagnetic coil is rigidly attached, and a nozzle linker rigidly attached to the coaxial cone member and threaded into the coil connector, the coaxial cone member being preferably made an integral part of the nozzle linker.
There are at least two prongs circumferentially attached to the body, which prongs are preferably made integral parts of the body. The prongs are provided with external thread on their ends distant from the body.
The bottom of the connector is made with bores, the number of the bores corresponding to the number of the prongs. The prongs extend into the connector through the bores, with a metallic blind screw threaded onto the threaded ends of the prongs. Between the blind screen and the coil, a spring is installed so that when the coil is energized, the blind screw is attracted to the coil compressing the spring and leading the throngs and the body. Due to that, the distance between walls of the bore and the coaxial conical member increases that causes pollutant particles clogging the outlet orifice to fall down the outlet orifice unclogging the orifice. It is also possible that the body is attached to the scrubber and thus made stationary, whereas the coil structure or a portion thereof such as armature moves toward the body shifting the conical member forward and thus increasing the bore.